1. Field of the Invention
The present invention relates generally to electromagnetic stirring of molten metal in continuous casting processes and, more particularly, is concerned with a counterflow electromagnetic stirring method and apparatus which generates counterflow stirring patterns that enhance mixing of the molten metal to produce an end product of improved quality.
2. Description of the Prior Art
Electromagnetic stirring (EMS) is utilized worldwide today as an aid in the production of continuous cast metals, such as steel and aluminum. The mission of EMS is quality improvement. The metallurgical benefits of EMS include: improved internal structure and cleanliness; and improved uniformity of composition and mechanical properties. EMS is applied at several locations in the continuous casting process: in the mold; below the mold; and in the final solidification zone. The stirring forces are caused by the interaction of applied magnetic fields and induced currents in a manner analogous to the operation of conventional induction motors. Depending on the type of strand being cast--billet, bloom or slab--the motor has a different physical configuration. The motions of the electromagnetic field produced by various EMS apparatuses heretofore have been rotary, linear or helical. In general, the EMS apparatuses are operated on AC either at line or at a reduced frequency, often below 10 Hz, although some DC apparatuses have been used.
For a description of the historical development of EMS, attention is directed to the following publications: "Design of Electromagnetic Stirrers For Continuous Casting" by Richard D. Nathenson et al, Iron and Steel Engineer, September 1986, p. 36; "Electromagnetic Stirring: Stepping Stone To Improved Continuously Cast Products" by H. S. Marr, Iron And Steel International, 1979, p. 29; and "EMS on Billet, Bloom And Slab Continuous Casters: State Of The Art in 1982" by J. P. Birat et al, 4th. International Congress of I.S.I., London, May 12, 1982. Representative of the prior patent art are the EMS apparatuses disclosed in U.S. Pat. Nos. to Dreyfus et al (2,573,319), Pestel et al (2,963,758), Taylor (3,235,243), Campbell (3,621,103), Von Starck (3,656,537), Hammarlund et al (3,709,476), Babel et al (3,804,147), Middleton et al (3,811,490), Ito et al (3,952,791 and 4,030,534), Sasaki et al (4,158,380), Birat et al (4,178,979), Zavaras et al (4,200,137), Delassus (4,321,958), Fujiwara et al (4,406,321), Winter et al (4,434,837 and 4,457,355) and Mulcahy (4,454,909), and in European Pat. Appln, No. 165,793 of Rostik et al.
Rotary electromagnetic stirring (REMS) apparatuses, along the lines of those disclosed in the above-cited Mulcahy patent and Rostik et al patent application, are widely used at the present time in the continuous casting industry. Such REMS apparatuses have been designed in various forms but all work on the same basic principles of electromagnetic induction. Basically, the apparatuses all contain a system of current-carrying conductor coils which produce a magnetic field moving in a direction transverse to the direction of molten metal travel through the continuous casting mold. This field penetrates the molten metal inducing electrical eddy currents which interact with the magnetic field to produce rotational stirring forces in the moving metal oriented transverse to its direction of flow. The metal attempts to follow the moving magnetic field and is thereby rotatably stirred.
More particularly, in the REMS apparatuses of the Mulcahy patent and Rostik application, a system of multi-phase excitation coils are typically wound about the mold and connected together in a manner very similar to the windings and connections of stators in conventional induction motors. Such coils form magnetic fields which move together to cause a continuous rotational motion of the molten metal in a common direction about the axis of the mold and transverse to the metal flow direction through the mold.
The continuous circular flow patterns induced by these REMS apparatuses cause the metal to circulate in a well ordered pattern, but do little to reduce the thermal gradients and material segregation that exist as the material cools from all outer surfaces inward toward the center of the mold. As a result, the rotary stirring of the metal produced by the Mulcahy-type REMS apparatuses is suboptimal in the sense that it fails to provide an end product having the high quality desired.
Consequently, a need still remains for a fresh approach to molten metal EMS in a continuous casting mold which will overcome the problems left unresolved by conventional REMS apparatuses such as disclosed in the Mulcahy paten and Rostik et al application.